Scramjets are air-breathing engines capable of travelling between five and 12 times the speed of sound. Although scramjet technology has been known to work since the 1950s, it has proven difficult to achieve in practice.

Dr Sean O'Byrne, lead researcher in the UNSW@ADFA laser diagnostics group, says one of the problems is how to maintain combustion in the supersonic air flow because the fuel must be injected, mixed, ignited, and burned within a few thousandths of a second.

"[Aeronautical engineers are] just at the point where they've proved scramjets work, now there is a lot of engineering to be done," he says.

Providing a kick start

Because the scramjet requires extremely high speeds of air to flow through the engine to work, it requires another form of engine to first get it to that velocity.

The problem then is how to create a 'spark plug' that will keep working at such high speeds.

O'Byrne says previous approaches have effectively used a giant arc like the one in an arc welder that uses a whole lot of energy to ensure the flame would not go out.

"But you may use as much power to produce the spark as you generate for thrust, so it is not really efficient," says O'Byrne.

Instead O'Byrne and PhD student Stefan Brieschenk, have focused a high-powered laser on the fuel jet, which creates a precise spark in the hydrogen fuel.

It does this through a phenomenon known as cascade ionisation, which occurs when the high-intensity beam literally "rips electrons off" the hydrogen molecule, creating a plasma that is the "spark".

The spark has peak temperatures approaching 100,000°C, compared with a conventional electrical spark of a few thousand degrees Celsius. At this temperature the hydrogen molecules are torn apart and they become highly reactive and accelerate the reaction between the fuel and air.

The researchers proved the concept in a hypersonic wind tunnel by creating conditions where hydrogen would not burn, and then using this approach to ignite it.

Importantly, says O'Byrne, because each laser pulse lasts for a few nanoseconds it requires less energy than a continuous electrical spark.

He says the laser-ignition may also have applications in generating greener energy through improved ignition of gas-fired engines for power generation.